Having the ability to have texture packs on servers. The way it will work is that it will use the way the server names the textures and the client just loads the local texture packs version if it matches. If it can't find a texture that the server has it uses the server's version as a fall back. The reason I thought of this is because I play minecraft and the texture pack that I use is at http://bdcraft.net and we might be able to have them try and make something for the community if we can get this working.

That will swap out the tileset, ui, and icon assets with the alternate style that is currently being developed.

You can also do the same with the sprite assets as they are in a separate mod, but we haven't yet begun to create the alternate implementations for them yet.

As far as the ability to override on the client-side with your preferred tileset / sprites: this is something that I have not given a whole lot of thought to, however I can see the merit in allowing something like this. I'll log this as a feature request for future reference.